Just pretend that the ground for the multimeter was hooked to the power source the whole time, thank you in advance. I promise this works.
30 power in from fused source. Only need to support swt signal requirements, which is usually pretty low amps
86 looped to 30
85 swt grd signal in.
87 swt power signal out
...
https://www.youtube.com/watch?v=xQMJmG57l1o